Modern automotive braking systems may be grouped into two basic categories, disc brakes and drum brakes. Of the two systems, disc brakes offer higher performance, simpler design, lighter weight, self-adjustability, and better resistance to water interference. Drum brakes have a greater number of parts than disc brakes and are therefore more difficult to service, but they are less expensive to manufacture, can easily incorporate an emergency brake system, and provide adequate braking force.
When a forward-moving vehicle brakes, the pitching motion of the vehicle creates a dynamic shift in the vehicle weight toward the front wheels. Therefore, it is necessary to have a highly effective braking system located at the front wheels of the vehicle. Accordingly, many of vehicles produced today have disc brakes on the front wheels and drum brakes at the rear wheels. Almost certainly, for as long as there have been braking systems in general use, there have been objectionable noises produced by these systems that engineers have attempted to eliminate.
The main components of a disc brake system are the rotor or disc, caliper, piston, and pads. The brake pads have a frictional lining supported by a rigid backing plate. The caliper holds the brake pads in proximity to the rotor and has at least one integrally mounted piston. Upon activation of the braking system, the piston urges the pad against the brake rotor thereby creating the frictional force necessary to slow the vehicle. Disc brake systems can further be subdivided into two subgroups, the floating-type caliper and the fixed-type caliper. The floating-type caliper contains at least one piston that presses the brake pad firmly against the rotor upon activation of the braking system. This movement creates a reaction force that causes the caliper to slide on pins thereby bringing the second brake pad into contact with the brake rotor. The fixed caliper design contains at least two pistons, one on each side of the rotor, each of which urges their respective brake pads into contact with the brake rotor while the caliper remains in a fixed position. The floating caliper system is the most widely used system on modern vehicles due to their lower cost and higher reliability relative to that of fixed calipers.
Both fixed and floating caliper disc brake systems may suffer from an objectionable noise termed “brake squeal” when a braking force is applied. This condition, especially at high frequencies, occurs whenever two or more of the brake components match in their dynamic behavior and couple together as a new system. In most cases, the brake pad resonances match with those of the brake rotor both in frequency and in wavelength. As a result, the brake pad will begin to vibrate in-phase with the rotor as a new system with very little damping. If the level of damping in the new system is lower than necessary to dissipate the input energy from the friction forces during braking, the amplitude of vibration of the new system will increase until the system becomes unstable leading to “brake squeal”. Therefore, by increasing the damping in the newly coupled system, the system can be maintained in a stable condition since it can dissipate more energy than is being introduced from the frictional forces. Since both the rotor and pad are vibrating together in-phase, the addition of damping to either component will tend to damp the system. However, due to the high temperature of the rotor in operation, many of the applications have been limited to adding damping to the pad.
Many inventors have attempted to alleviate the noise problem that may be encountered with disc brakes.
U.S. Pat. No. 5,660,251 issued to Nishizawa et al. on Aug. 26, 1997, discloses a disc brake damping mechanism that detects vibrations of the brake rotor by a piezoelectric element pressed against the backing plate of one on the brake pads. The detection signal is input to a control circuit, which then applies a control signal to another piezoelectric element that produces oscillations having a frequency operable to reduce the detection signal to zero. This active damping system may be more costly to implement than that of a passive system, and may not be economically viable for large-scale use on commercially produced vehicles.
U.S. Pat. No. 5,099,962 issued to Furusu et al. on Mar. 31, 1992, discloses a disc brake backing plate with two layers of viscoelastic material disposed between three metal plates forming a constrained layer viscoelastic laminate to form a brake pad isolator or shim. Although constrained layer damping treatments have been found to be effective, in most cases there is still a need to introduce additional damping to the system.